The present invention concerns blends of poly(tetrafluoroethylene) (PTFE) and poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) (PFA) melt spun into fibers.
There is extensive literature on blends of PFA and PTFE. However, the vast majority of the patent and literature work involves dispersion blends where no effort has been made to produce homogeneous melts. Further, the bulk of the work involves commercial, high molecular weight PTFE materials. Typical PTFE polymers that are used to form useful articles are of extremely high molecular weight, of the order of 107. The high molecular weight of these homopolymers creates difficulties in forming fibers via melt processes due to their high melt viscosities. On the other hand PTFE grades with viscosities low enough to be melt processed (less than 105 Pa-s) do not exhibit useful strength as formed articles.
The conditions under which PTFE and PFA may cocrystallize is not clear in the open literature. U.S. Pat. No. 5,473,018 (Namura et al.) discloses certain blends of PTFE with PFA. The tensile strength of molded bars is measured therein. Restrictions were placed on the crystallization temperature and heat of crystallization of the PTFE (essentially requiring the PTFE to be below some threshold molecular weight). The claims of this patent concern only blend compositions containing  less than 4% by weight PTFE. This reference also discloses blends of up to 50% PTFE content. Properties of blends of higher PTFE content were not useful in the context of this patent. Namura et al. reported that the tensile strength of molded bars of a 50% PTFE blend reported was approximately 50% of a molded bar of PFA alone. Fibers made from such blends are not mentioned.
The articles xe2x80x9cCrystalline Homopolymer-Copolymer Blends: Poly(tetrafluoroethylene)-Poly(tetrafluoroethylene-co-perfluoro-alkylvinyl etherxe2x80x9d, J. Runt, L. Jin, S. Talibuddin and C. R. Davis, Macromolecules, 28, 2781-2786 (1995) and xe2x80x9cPhase Behavior of Crystalline Blends of Poly(tetrafluoroethylene) and of Random Fluorinated Copolymers of Tetrafluoroethylenexe2x80x9d, R. Pucciarello and C. Angioletti, J. Polymer Science: Part B: Polymer Physics, 37, 679-689 (1999) describe blends of PTFE and PFA. Runt et al. discovered that PTFE and PFA mixed in aqueous dispersions of the two components crystallize separately under most crystallization conditions investigated and concluded that PFA and PTFE may cocrystallize only under rapid crystallization conditions. In the more recent Pucciarello et al. article PTFE and PFA, under their conditions which utilized dry powder mixing, do not cocrystallize. Pucciarello et al. concluded that the more intimate mixing of the particles of Runt et al. would allow, only in extreme conditions (quenching) cocrystallization of PTFE and PFA.
WO 00/08071 (Smith et al.) concerns blends of PTFE and mentions blends of PTFE with certain copolymers of PTFE. All examples in Smith et al. involve blends of two or more different PTFEs.
The present invention concerns a fiber, comprising a composition comprising a blend of at least one poly(tetrafluoroethylene) and at least one poly(tetrafluoroethylene-co-perfluoro-alkylvinylether).
The present invention also concerns a process of forming a fiber, comprising the steps of: (a) contacting at least one poly(tetrafluoroethylene) with at least one poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) to form a blend; (b) heating the poly(tetrafluoroethylene) above the melting point of the poly(tetrafluoroethylene) prior to, simultaneously with, or subsequent to contact with the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether); (c) heating the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) above the melting point of the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) prior to, simultaneously with or subsequent to contact with the poly(tetrafluoroethylene); and (d) extruding the molten blend through a die to form a fiber comprising a composition comprising a blend of at least one poly(tetrafluoroethylene) and at least one poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether).
The present invention further concerns a process of preparing a cocrystallized blend comprising the steps of (a) contacting at least one poly(tetrafluoroethylene) with at least one poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) wherein the viscosity of the poly(tetrafluoroethylene) is within two orders of magnitude of the viscosity of the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether); (b) mixing the poly(tetrafluoroethylene) and poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether thoroughly to form a blend; (c) heating the poly(tetrafluoroethylene) above the melting point of the poly(tetrafluoroethylene) prior to, simultaneously with, or subsequent to contact with the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether); and (d) heating the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) above the melting point of the poly(tetrafluoroethylene-co-perfluoro-alkylvinyl ether) prior to, simultaneously with or subsequent to contact with the poly(tetrafluoroethylene).